Metal-bearing ores occur often in nature as oxides, such as iron oxide, chromium oxide, vanadium oxide, titanium oxide, aluminum oxide, and silicon dioxide. The most obvious attempts to directly reduce these ores by contacting them with hydrogen have been impractical due to the cost of pure industrial hydrogen, and the high temperatures involved for the reaction. Processes of this type are typically conducted in chambers isolated from the oxygen of the air. One approach has been to use synthesis gas, a mixture of H.sub.2 and CO obtained by reacting steam and carbon (or a hydrocarbon) at high temperature. Reaction of the hydrogen with the oxygen in the ore produces water, and the reaction of the carbon monoxide with the oxygen in the ore produces carbon dioxide. Another approach has been to use methane as a reducing gas temperatures in the order of 950.degree. C. to 1200.degree. C., as taught in U.S. Pat. No. 4,268,303. Using this approach on iron ore, moving-bed reactors and expensive heaters are needed, methane pyrolysis and carbon deposition takes place, and sintering of the iron particles into agglomerates is a problem. The operation of a moving-bed reactor with three zones is described in U.S. Pat. No. 4,556,417, which operates at only 900.degree. C. to 960.degree. C. with natural gas, and which avoids agglomerates. Considerable heat input is still necessary.
The use of an argon inert-gas plasma torch, projecting a stream of high-temperature argon ion onto the top surface of a crucible containing ore particulates, and with the injection of neutral methane gas onto the heated surface by means of a separate water-cooled lance, was described by Vogel et al. (Steel Research, 60, pp. 177-181 (1989)). This relies upon argon ion energy transfer to the neutral methane above the particulates, argon ion and argon neutral energy transfer to the exposed top layer of the particulates in the crucible, and the thermodynamics of the reduction reactions at high temperatures. The kinetics of oxygen removal are constrained by the mixing of unreduced particulates and reduced metal. It was not possible to reduce pure titanium oxide. Since this process took place at nearly atmospheric pressure, there was considerable energy loss from the argon arc, via the ambient gas neutrals, to the walls of the chamber as well as to the crucible. The use of a plasma torch to effect high temperatures, 6000.degree. K. to 10,000.degree. K., which can be used to melt refractory metals, to transfer energy to the surfaces of particulates, and to cause particulates to fuse to one another or to adjacent cold surfaces is well known. Such plasma torches are generally operated with a pure feed gas, such as argon, although the use of hydrogen can be envisioned provided that the ionic attack of plasma torch electrodes can somehow be avoided. The use of hydrocarbon-containing gases in a plasma torch is employed for the production of acetylene and/or carbon black, as the plasma energetics cause cracking of the gaseous hydrocarbon feedstock. A method is described in U.S. Pat. No. 5,105,028 for using a plasma torch and a feedstock gas mixture including a hydrocarbon-containing gas and a hetero-atom containing gas, ionizing the mixture, and creating a more complex compound such as an alcohol or phenol, by ion interaction in the plasma. The electric arc is also described in U.S. Pat. No. 4,566,961 as providing the energy for a process of combination of high molecular weight carbonaceous material with the hot gases containing C.sub.1 -C.sub.4 saturated hydrocarbons from the arc, so as to produce low molecular weight hydrocarbon products. This process is operated at high, near-atmopsheric pressures, and requires considerable input power due to the energy transfer by ambient neutrals to the walls of the enclosure. Moreover, the probability of repetitive interaction of the particulates (if coal is the type of high-molecular-weight carbonaceous feedstock used) with the available C.sub.1 -C.sub.4 hot gases, is limited.
The use of a low-density plasma containing hydrocarbon ions and methyl ions, inside of an array of tubular elements, is described in U.S. Pat. No. 5,019,355, as a method for the production of higher hydrocarbons by ion-impact-stimulated chemical reactions at the interior surfaces of the tubular elements. Another U.S. Pat. No. 5,141,715 describes the use of a plasma and a set of electrode arrays to accomplish conversion of compound gases into other compound gases. However, the use of particulates in such apparatus is problematic, as the particulates would accumulate and clog the apertures or tubular elements described, impairing their function.